The transmission of communication signals for voice, video, data, and the like, is increasingly accomplished using optical fibers because of the high bandwidth and throughput capabilities they offer in comparison with conventional electrical conductors. Unlike connections for electrical conductors, however, the fiber optic connections must be executed with great care and precision in order to minimize losses in the transmitted signal. As is known in the art, two optical fibers are connected by bringing the end faces of the optical fibers into coaxial alignment such that the optical fiber end faces abut or are separated by only a slight distance. In this way, the two optical fibers form a substantially continuous waveguide to transmit signals. Typically, each optical fiber is mounted in a passageway (e.g., a bore, channel, groove, or any other similar structure) formed through a ferrule, which may be a cylindrical or non-cylindrical shaped body made of material, such as, ceramic or plastic.
The ferrule is usually mounted in a body of an optical fiber connector that is configured to mate with another connector also having a ferrule mounted therein. The connectors are configured so as to coaxially align the ferrules and the optical fibers therein. For example, an SC ferrule may be configured such that the bore in each ferrule is nominally centered, relative to the outer surface thereof.
The degree of precision with which the optical fibers are aligned with each other affects the performance of the connection between two optical fibers. Several factors can affect the loss or attenuation of signal caused by the connection including: (1) lateral displacement of the mating end faces of the optical fibers, that is, the lateral distance between the two axes of the optical fibers at the mating end faces thereof; (2) angular misalignment of the optical fibers; and (3) axial separation between the end faces of the optical fibers. Other factors that can affect the loss or attenuation of signal at a fiber-to-fiber interconnection include: index of refraction mismatch, mode field radius mismatch, the shape and finish of the end faces, and physical damage that may be present at the end faces.
Of these factors, lateral displacement and angular misalignment have significant impact on the signal attenuation. Lateral displacement or offset of the optical fibers of two mated connectors can result from various causes. Two important causes are: (1) eccentricity of the passageway of the ferrule relative to the ferrule's outer surface; and (2) offset of the optical fiber within the passageway. With regard to the latter, the passageway in the ferrule typically is slightly larger in diameter than the optical fiber, and hence, until the optical fiber is fixed in position in the passageway via an adhesive, the optical fiber is free to move in the passageway. Accordingly, the eccentricity of the optical fiber relative to the ferrule can be higher or lower depending on where the optical fiber is secured in the passageway.
It is known to take advantage of this ability of the optical fiber to move in the passageway so as to minimize the eccentricity of the optical fiber in a ferrule whose passageway is not perfectly centered or aligned in the ferrule. In general, even when care is taken to try to form the passageway in the exact locations of the ferrule (e.g., center of the ferrule), the passageway is usually offset from the central axis of the ferrule to some extent. The offset, or eccentricity, of the passageway is generally characterized by two parameters, the magnitude of the offset and the direction of the offset, both parameters being measured at the end face of the ferrule.
For example, the eccentricity of a ferrule having a central bore may have a magnitude of 1 μm and this offset may be in the direction of a radial line that can be designated as the 0° position. As noted above, it is known to minimize the eccentricity of an optical fiber disposed in an eccentric passageway by positioning the optical fiber in a particular direction in the passageway. Thus, for instance, in the example given above, the optical fiber can be positioned to one side of the passageway in the direction of a radial line that is displaced 180° from the radial line along which the passageway is offset. In this manner, the eccentricity of the optical fiber, which would be 1 μm if the optical fiber were exactly centered in the passageway, is reduced by half the difference between the diameter of the passageway and the diameter of the optical fiber. Thus, assuming for illustrative purposes that the passageway has a diameter of 126 μm and the optical fiber has a diameter of 125 μm, the eccentricity of the optical fiber can be as low as 0.5 μm if the optical fiber is positioned to the side of the passageway in the opposite direction to that in which the passageway is offset. In contrast, if the optical fiber were positioned to the side of the passageway in the same direction to that in which the passageway is offset, then the optical fiber eccentricity would be 1.5 μm.
U.S. Pat. No. 4,880,291 discloses an apparatus and method for positioning an optical fiber within a passageway of a ferrule in a predetermined orientation with respect to the direction of eccentricity of the passageway relative to the longitudinal axis of the ferrule. The apparatus has a plurality of receptacles or nests for receiving a plurality of connector bodies each having a ferrule with an optical fiber inserted in a bore thereof. The ferrule in each connector body is rotationally oriented such that the direction of eccentricity of the bore in the ferrule is diametrically opposite to the direction of a protruding tab formed on the outer surface of the connector body. Each nest in the apparatus has a keyway for mating with the tab on the connector body, such that the connector body is oriented in a known manner in the nest. The apparatus includes a plurality of wire-like bails that press against the optical fibers projecting from the ferrules so as to force the optical fibers to the side of the bores in the direction of the tabs on the connector bodies, thus minimizing the optical fiber eccentricity. An adhesive in the bores is then cured to fix the optical fibers in place.
Although a general method of pushing an optical fiber to one side of the passageway is thus known, the prior art does not provide any guidance on how best to accomplish the method. In practice, it has been found that pushing an optical fiber to a preferred position, such as, to one side of the passageway, can improve the eccentricity of the optical fiber at the end face of the ferrule. However, as illustrated in FIG. 1, which is shown in exaggerated detail for illustrative purposes, the force exerted on the optical fiber to push it to one side of the passageway can cause the optical fiber to bend or bow. When the end of the optical fiber is cut off and the end faces of the optical fiber and ferrule are polished, the optical fiber and ferrule material are removed for some axial distance back from the original end face of the ferrule, as illustrated in FIG. 1 by the broken line representing the position of the end face of the ferrule and optical fiber after polishing. As a result, the optical fiber position relative to the passageway after polishing can differ appreciably from the optical fiber position prior to polishing. In addition, attenuation is caused by the angular misalignment of the mating passageways. As illustrated in FIG. 1, due to the fiber bend, an angular misalignment of the optical fiber axis relative to the ferrule axis exists.
Thus, a need exists for a method and apparatus for positioning an optical fiber in a passageway to compensate for eccentricity of the passageway relative to the longitudinal centroidal axis of the ferrule such that the optical fiber position relative to the passageway is substantially straight for a distance back from the ferrule end face that is at least as great as the distance representing the maximum length of material that will be removed during polishing.